I have not seen that you have checked the IAT sensor. I also found it really helpful to check resistance and continuity at the computer's harness.

The best tool someone like you (long term owner of a CIS-E MB) can own is a wide band O2 sensor.

BTW, testing the idle microswitch is easy. I use a small piece of vacuum line wedged behind the lever arm. Stick it in there and drive the car a bit. Until I pulled and adjusted the fuel distributor, EHA and idle screw, the idle switch engagement took my 14.7 air fuel (AF) ratio (in gear at 1000 RPM) down to 9.1 (no change in RPM). Without idle switch, in P or N RPM went up to 2000 but AF went back to 14.7. I got 22 MPG with the idle switch out of the equation.

Since then I have adjusted the fuel distributor again. Going about 1/2 turn in on all six screws with corresponding adjustments to the EHA and idle screw brought the idle back down with the idle switch back in the loop. I need to adjust again since it still is not perfect. Once done I will update the CIS-E tuning thread I created here and added to your site.

Bottom line, without the wideband O2 sensor, I would have been guessing.

You can also pull your plug wires one by one with the motor running to verify that you are getting good spark and fuel at each cylinder.